In conventional semiconductor chip manufacturing techniques, photoresist is routinely used to define areas in the wafer surface that are to be etched or into which dopants are to be diffused.
According to conventional processes, a layer of photoresist is deposited on top of the semiconductor chip layer to be worked. Next, selected portions of the layer of photoresist are exposed to a light source containing a preselected wavelength. The exposed portions of the photoresist become soluble in a developer compound, while the unexposed portions remain insoluble. The developer compound is next used to strip away or "develop" the photoresist, thus defining areas in the chip surface for diffusion or plasma etching.
Although contact printing processes are known, the photoresist layer is often used with a projection printing process. According to this printing process, an opaque projection mask is first printed on a transparent plate to define the areas of the photoresist which are to be exposed. Ultraviolet light containing the selected wavelength is transmitted through the projection mask and an image reduction lens of appropriate power and is then projected on the photoresist layer.
As the critical dimensions of the imaged features of integrated circuits have grown smaller and smaller, diffraction around the edges of the features has grown more and more pronounced. The resultant diffraction patterns degrade imaging in that they produce photoresist-defined areas having wider dimensions and sidewalls with undesirable slopes.
This diffraction problem has conventionally been solved with a contrast enhancement layer placed on top of the photoresist layer. The contrast enhancement layer is initially opaque, but becomes transparent upon exposure to the same light that develops the photoresist. On the deposition of a sufficiently thick contrast enhancement layer, only the intended area of the photoresist will be exposed. Substantially perpendicular photoresist sidewalls result, which means that the size of an etched feature plasma etched thereafter can be more strictly dimensioned.
Conventional contrast enhancement compounds have a serious drawback in that they are not water-soluble. Typically, these compounds must be coated onto the photoresist with the aid of an organic solvent. Conventional photoresists are however also somewhat soluble in this organic solvent, and thus the contrast enhancement layer coating process may endanger the integrity of the underlying photoresist. The exposure time of the photoresist to the organic solvent must therefore be closely watched.
After the photoresist layer has been exposed, it is necessary to strip off the conventional contrast enhancement compound with further organic solvent, which will then again tend to attack the photoresist. Not only does the use of an organic solvent pose a problem in preserving the integrity of the underlying photoresist, it also introduces further steps, time and expense in the manufacturing process. Finally, the conventional contrast enhancement compound has been found to be thermally unstable and to have a poor shelf life.
A need has therefore arisen in the industry for a contrast enhancement composition that does not require an organic solvent, that simplifies the patterning process, and that has longer shelf life than previously known enhancement compositions.